This is basic stuff for Source and you don’t need to access any classes. You can even overlap them too if you want. You can set a column to be a fixed size and the other ‘columns’ will adjust their width for each breakpoint and this makes complex layouts a breeze. The 12 column grid dinosaurs can’t do any of this stuff.
I haven’t tried UIKIT, but certainly after using Source exclusively for the last few months for new sites, I have yet to find something that I can’t do with Source and going back to the other 2 Frameworks to maintain older sites has not not been very enjoyable.